Since high purity alumina porcelain is excellent in heat resistance, chemical resistance, mechanical strength, etc., and shows a small high-frequency dielectric loss, alumina porcelain is frequently used in a wide variety of arts. However, since the various properties of the porcelain, and particularly the electric properties thereof, may change significantly even upon slight changes of the kind and amount of additives such as sintering aids, etc., the porcelain is unsuitable for practical use as a high-frequency dielectric material which is utilized in the above described uses.
The inventors previously investigated various additives for utilizing alumina porcelains as such high-frequency dielectric materials, and previously proposed an alumina porcelain composition comprising alumina as the main component having added thereto definite amounts of CaO and TiO.sub.2, and an alumina porcelain composition comprising alumina as the main component having added thereto MgO and SiO.sub.2, as described in Japanese Patent Application No. 32113/84 (corresponding to Japanese Patent Application (OPI) No. 176967/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")) and Japanese Patent Application No. 125136/83 (corresponding to Japanese Patent Application (OPI) No. 16861/85), respectively.
Recently, there has been a trend that electrodes and conductive circuits for a microwave integrated circuits are formed as fine lines of thin layers on the polished surface of a base, and hence for preventing the occurrence of problems by cutting of the fine electrodes or circuits, a dense material wherein a pore is absent or if the pore is present the pore has very fine diameter is required for the base. However, the alumina porcelain compositions of the above described Japanese Patent Application Nos. 32113/84 and 125136/83 do not have a denseness as high as might be desired, and dense alumina porcelains which are fully satisfactory as high-frequency dielectric materials have not been heretofore known.